Plasma: pattern size for measurements - AVS Forum
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post #1 of 11 Old 09-15-2011, 02:06 PM - Thread Starter
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Hey eveyone,

I'd like to keep this nice and simple as it is confusing enough as it is. I have been quite unable to find a definitive way to go about this, so I'll dig into it myself pretty soon. Here goes:

1. How exactly do you calculate pattern size?
2. What is the difference between the APL (average picture level) patterns and the "regular" ones?
3. There has been talk that a problem is that patterns are always displayed within a black rectangle. Is this so?
4. Can someone describe how we can detect when the ABL (automatic brightness limiter) kicks in?
5. Could someone link the term "stimulus" to this? I understand it ranges from 0 to 100. IRE is a synonym for it? Is there some correlation between stimulus and pattern size?

Many thanks. I hope finding the answer to these questions serve more than myself

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post #2 of 11 Old 09-15-2011, 05:56 PM
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Quote:
Originally Posted by Jeroen1000 View Post

1. How exactly do you calculate pattern size?

Window size is usually discussed as percent area. Take the height of the window times the width of the window, and divide by the height of the screen times the width of the screen. That will give you a decimal, which could be multiplied times 100 for a percentage.

(H1*W1)/(H2*W2)*100 = percent area

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2. What is the difference between the APL (average picture level) patterns and the "regular" ones?

The APL patterns from AVS HD 709 are similar to bar patterns, but they're easier to measure. All the video levels for measurements are on-screen at the same time, so generally average brightness of the image is expected to remain constant for all the measurements. With the APL patterns the intent is to have a constant average brightness during measurements.

A typical window usually has only two video levels, the level to be measured and the surrounding black. Usually the center area of the window remains the same size, so as the video levels increase in brightness from black to white the average brightness of the image also increases. With typical windows or fields the average brightness of the image varies depending on the video level being measured.

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3. There has been talk that a problem is that patterns are always displayed within a black rectangle. Is this so?

Not really, the main difference between the patterns has to do with average brightness. The APL patterns have a constant average brightness, and with standard windows or fields average brightness varies. Various displays react differently as average brightness changes (see Note). Some displays will deliver a similar light output regardless of average brightness, and other displays will vary light output as average brightness changes. Here are two video patterns to quickly look at how average brightness may affect the image:

1) AVS HD 709 Misc. Patterns A5, Dynamic Brightness
2) Avia II "Black Level Bars + Steps + Varying Gray" pattern

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4. Can someone describe how we can detect when the ABL (automatic brightness limiter) kicks in?

Probably the easiest thing to do would be to measure the display response, but I get the impression not many people have done that. There are a couple ways you could look at how the display deals with a constant video level and a changing average brightness. One way would be to use a constant measurement area and then increase the brightness of the background (the Avia II pattern mentioned above follows this concept). Another way would be to measure what happens as the area of the measurement pattern increases. For example ColorHCFR will let you display windows of various sizes, and you could measure what happens to Y as you increase the window from a very small area to a larger area.

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5. Could someone link the term "stimulus" to this? I understand it ranges from 0 to 100. IRE is a synonym for it? Is there some correlation between stimulus and pattern size?

The percent markings on the patterns just indicate video levels. The video level is independent from pattern size. You can have a white video level that is used for a very small window, and you can have a white video level that is used in a very large window. The small window size has a lower average brightness than the larger window, although the two windows use the exact same video level.

The patterns above marked 1 and 2 have some bars on the screen that always use the same video levels. Part of the screen changes shade in those patterns, which causes average brightness to change. The bars from the mentioned patterns may change light output (change brightness) as a result of the average brightness changing, even though the video levels have remained the same. This sort of change in light output independent from video level is the main disconnect between gamma theory described by Charles Poynton and typical application.


Note: In fact some displays work in rather opposite directions when it comes to how they deal with average brightness. Lets say you have a Samsung LCD with dynamic backlighting and you also have a Samsung plasma. Both TVs are showing the same picture with some white in the image. Lets say the image is from a camera on a train, the white is from a light on a train, and the train drives into a tunnel. So you have some white in the image and the scene transitions between a somewhat bright scene and a dark scene. The white on the LCD may get darker on the transition to the dim scene, while the white on the plasma might get brighter as the scene gets darker. In both cases you have the exact same input (video levels), but how the displays react is not necessarily similar.
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post #3 of 11 Old 09-16-2011, 04:49 AM - Thread Starter
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Thank you, this is a very comprehensive reply. I'll have to reread a couple of times to let it all make absolute sense. The numbers 75 and 100, are often used to indicate a feature of a pattern. Is this number directly referring to the stimulus (video level)? I believe so, but, to which level would it refer? (seeing we have 16-235). 100 most likely refers to level 235 but for 75 this is not so clear cut.


I also feel there is no censensus (or is there 1?) to whether one should calibrate with the ABL in mind or not (let it kick in vs make sure it does not kick in). From the thread I have read, it seems like the best pattern size lies around the 10-12% mark. It also seems that a normal window pattern is used (so a pattern with only 2 video levels as you have just explained).

edit: found some explanations in the AVS manual. I also found the pattern you mentioned for checking whether the APL pulls tricks (it's A5 instead of A4 but that's likely just a typo).

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post #4 of 11 Old 09-16-2011, 05:51 AM
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Quote:
Originally Posted by Jeroen1000 View Post

The numbers 75 and 100, are often used to indicate a feature of a pattern. Is this number directly referring to the stimulus (video level)? I believe so, but, to which level would it refer? (seeing we have 16-235). 100 most likely refers to level 235 but for 75 this is not so clear cut.

I'll guess you found the percent definition in the PDF.

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I also feel there is no censensus (or is there 1?) to whether one should calibrate with the ABL in mind or not (let it kick in vs make sure it does not kick in).

I'll let other people comment on this, because it's not my main concern. My main concern is that the typical way of measuring gamma with windows causes average brightness to vary, and different displays simply don't tend to vary similarly as average brightness changes. What I'm primarily commenting on is how identical measurements don't necessarily indicate similar display performance when measuring window patterns.

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From the thread I have read, it seems like the best pattern size lies around the 10-12% mark.

My question would be, 'best' based on what?

Quote:
it's A5 instead of A4 but that's likely just a typo

Yes, I had the wrong pattern number listed.
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post #5 of 11 Old 09-16-2011, 08:48 AM
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As mentioned in other posts, pattern size is expressed as a % of screen area.

IRE is NOT a synonym for ANYTHING. And IRE does not exist in the digital video world AT ALL. Any use of "IRE" while referring to digital video is just wrong - but old-timers can't let go of it. IRE describes the relationship between ANALOG video signal voltage levels and screen brightness. Since digital video uses 1s and 0s to to represent screen brightness levels IRE does not apply. Someone who knows what IRE is and who is using it correctly could easily confuse someone who thinks IRE and % white are the same thing.

% White and Stimulus are also NOT THE SAME THING. For example, 75% white (or 75% colors) are roughly 50% stimulus in the world of digital video where gamma is typically in the range of 2.2 -2.4. If your display generates 30 fL for 100% white, 75% white will produce about 15 fL (this will vary somewhat depending on gamma). 15 fL is half as much light as 30 fL so 15 fL would be 50% stimulus.

If there was NO gamma in video displays (linear light output), % white and stimulus % would be the same thing. When gamma is used, the curve connecting the white point and black point is NOT a straight line. The shape of the gamma curve is what determines the relationship between % white and stimulus %. Change gamma and you change the relationship between % white and stimulus %. You really don't need to worry much about stimulus % for video calibration purposes. Stimulus % is easily calculated after you measure the grayscale in 10 steps or more. Simply divide the measured fL for each grayscale step by the 100% fL.

Every plasma model is different re. how the brightness limiting works - in so far as when it starts and how much limiting there is. It is what it is. You measure a display and you COULD graph it... but why bother? You can't change it. The brightness limiting is really a non-issue for calibration - you calibrate a plasma like you calibrate any other TV. But the brightness limiting, for my money, is best dealt with if the window pattern size is about 12% of the screen area. Some discs and generators have windows that are about 16% of the screen area... IMO, that large of a window is about right for CRT but is a little too large for plasma -- both have brightness limiting but CRT doesn't do it as much as plasma does. You will change the calibration somewhat by changing the window size (for plasma displays). How much it changes the calibration depends on how much you change the window size. A calibration with full-screen patterns would be quite different from a calibration with 12% sized windows. And again, IMO, the 12% window size will give you the best calibration compromise for plasma, though I wouldn't get too excited about window sizes between 10% and 14% of screen area... the result within that range will be pretty similar. Even 16%-sized windows wouldn't throw a plasma calibration off enough to worry about.

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post #6 of 11 Old 09-16-2011, 01:00 PM - Thread Starter
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I know Doug is on the mark about both IRE (I kind of knew about IRE and I feel a bit guilty using it wrongly) and stimulus. I remember googling these terms for hours and found references by very respected members (like himself) telling the same. I can clearly remember that stimulus is not the same as % white. I believe stimulus even differs from disc to disc (perhaps not for 100% white and 100% black though). Or that some much used disc had a fault of some kind. Unfortunately, I cannot remember the specifics.

Well, I'll try not to worry too much, as long as I like the picture. Heck, I like the uncalibrated picture too (only contrast and brightness have been set correctly). I do see a little red push, but it isn't that bad it bother me even second of the day.

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post #7 of 11 Old 09-16-2011, 05:30 PM
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On my VT30, I calibrated all 3 color temps in the service menu (cool, normal, warm) using 3 different patterns: small APL, large APL & standard window patterns. Using this technique, I was able to cycle between the different color temps in the user menu while watching various content to see which one yielded the best result visually. On the VT30, this seemed to be the large APL windows by a long shot.
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post #8 of 11 Old 09-17-2011, 02:39 AM
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I would gather that APL patterns are best used when setting up saturation and phase, as pro equipment usually rely on SMPTE colour bars to align the 'blue only mode'
However, greyscale adjustments are probably best done with static greyscale patterns at 5ire intervals.
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post #9 of 11 Old 09-17-2011, 01:42 PM
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Originally Posted by delphiplasma View Post

However, greyscale adjustments are probably best done with static greyscale patterns at 5ire intervals.

This was not the case on my VT30. Large APL gave the most accurate result visually after comparing directly to small APL and standard window patterns.
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post #10 of 11 Old 09-17-2011, 10:36 PM
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Quote:
Originally Posted by jkozlow3 View Post

On my VT30, I calibrated all 3 color temps in the service menu (cool, normal, warm) using 3 different patterns: small APL, large APL & standard window patterns. Using this technique, I was able to cycle between the different color temps in the user menu while watching various content to see which one yielded the best result visually. On the VT30, this seemed to be the large APL windows by a long shot.

This is simply incorrect. If by 'large APL windows" you really mean "large windows" in regard to how much of the screen the window occupies, by using large windows you NEVER calibrate anything bright since large window size will limit the brightness... for example... a 100% (full field) might measure 17 fL for 100% white while a window covering 12% of the screen area could measure 35 fL without changing ANYTHING but the window size. With the large (full screen in this case) window size, you'd never measure a 100% white level higher than 17 fL so everything from 17 fL to 35 fL would be uncalibrated. Those higher levels would exist most of the time in most scenes since any small bright white area would be well above 17 fL.

As I posted before, you want a window covering about 12% of the plasma panel for best calibration results (best range 10% - 14% window size). Go larger than that and you'll never measure the highest brightness levels the panel can achieve.

I've seen arguments for using windows covering only 5% or even less than that for plasma because those produce even higher peak white levels, but you have to compromise SOMEWHERE - that's where the 12% window size comes into play... that's the best compromise... for plasma.

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post #11 of 11 Old 09-18-2011, 09:48 AM
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Originally Posted by Doug Blackburn View Post
If by 'large APL windows" you really mean "large windows" in regard to how much of the screen the window occupies
It's simply a non-standard pattern that is similar to the idea of measuring a typical bar pattern. The video level is moved to the middle area to make measurements more convenient, and to avoid non-uniformity of certain display types such as projectors. The point of the pattern is merely to measure a series of video levels while holding a constant overall screen brightness, like would happen if someone measured a gray bar pattern on a display with a uniform screen such as plasma. This practice of maintaining a constant overall screen brightness during measurements is generally in line with theoretical discussions of gamma, since gamma theory generally avoids practical concerns such as 'brightness limiting'.

Consumer calibration has instead generally adopted the idea of measuring video levels using window patterns, and there is no actual standardization for window size (area). So consumer calibration essentially uses window patterns of an undefined size to try to make various displays look similar. This practice appears to present issues with gamma since overall screen brightness varies as video levels change, and different display models simply do not react similarly as overall screen brightness changes (see note). The end result of the standard practice of using window measurements is that identical gamma (relative Y) values do not necessarily indicate similar performance in side-by-side comparisons. This typical practice also does not follow usual theoretical discussions of gamma, since such explanations rarely touch on differences in display performance.


Note: The displays that tend to vary gamma measurements the most depending on overall screen brightness likely fall into the categories of dynamic backlight and adjusting iris models, but even LCD with fixed backlighting varies in how it measures light output (Y) compared to plasma when using windows.
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